This invention pertains to apparatus for granulating plastic strands.
The granulating of plastic strands by means of cutting knives immediately upon the emergence of the strand, referred to, as such, also as head granulation, requires a sensitive temperature control of the plastic strands within the nozzle plate all the way to the outlet for the strand. The temperature control must be adapted to retain the desired or predetermined viscosity of the individual molten strands up to their place of emergence, an increase in viscosity upon the emergence of the strand being necessary in order to obtain optimum cutting conditions.
In known granulating devices, this temperature control is obtained essentially by a corresponding addition of heat within the nozzle plate or by special insulating measures on the nozzles, or by the direct use of a coolant in the region of emergence of the plastic strands.
The processing of certain plastics which have a narrow softening range between the liquid and solid states, as is true, for instance, of polyamides and polyesters, requires a precise cooling of the plastic strands for the cutting process, without impairing the temperature conditions in the nozzle plate.
U.S. Pat. No. 3,792,950 shows a granulating device of the type indicated in which nozzle inserts, formed of porous material, in the nozzle plate directly adjoin the conically tapering inlet channel of the nozzle bore and are each individually surrounded by an annular channel for the introduction of a coolant into the nozzle bore.
Since in this case heat is removed by the action of the coolant on the inlet-channel insert, this action must be counteracted again by sufficient heating of each individual inlet channel insert by a suitable arrangement of heating channels. Thus, a large radial separation of the nozzle boreholes is required. Furthermore, the fine-pore nozzle inserts very easily become clogged, so that only carefully filtered pressurized water can be used, which means a high expense, in addition to which the water pressure thereof must be adapted to the melting pressure in the feed region.